Processes used to treat the wastewater discharged from primary and/or secondary treatment processes are referred to as “advanced” treatment systems. Advanced treatment systems reduce the biological nutrient content in the wastewater. One of the most significant biological nutrients is nitrogen.
Nitrogen-containing species, a common byproduct of the decomposition of organic matter, can be an environmentally controlled substance. By way of example, ammonia (NH3) not only imposes an oxygen demand on the water in which it is discharged but also at high enough concentrations can be toxic to aquatic life. Nitrite (NO2) can be toxic if ingested by vertebrate forms of life. Nitrate (NO3) can be toxic to living organisms if ingested at high enough levels. In particular, nitrate is believed to be toxic to infants through a condition known as infant methemoglobinemia. Under the chemical conditions unique to an infant's stomach, nitrate is converted into nitrite. Nitrite interferes with the role of hemoglobin in the respiratory and metabolic functions. Finally, nitrogen-containing species can act as a plant nutrient, stimulating undesirable growth of algae and other aquatic plant life.
A typical wastewater disposal system for treating nitrogenated wastewater from a septic tank includes an outlet pipe leading to a dispersal manifold. The dispersal manifold includes a collection of porous pipes through which the wastewater seeps out. The manifold disperses the wastewater over a bed of soakaway material or leachfield, such as gravel/sand. The ammonium, present in the septic tank, reacts under the aerobic conditions prevailing in the soakaway to nitrate, which then passes through the soakaway into the groundwater.
Removal of nitrogen from septic tank wastewater is actually a two step process. The first step, called nitrification, requires conversion, in the soakaway, of the nitrogen in ammonia to nitrogen in nitrates. This is accomplished by the use of aerobic bacteria; nitrosomonas and nitrobacter, that are already present in domestic wastewater. These bacteria grow and multiply in the presence of dissolved molecular oxygen at concentrations greater than about 1 milligram per liter. The bacteria convert ammonia nitrogen into nitrate nitrogen. The second step, called de-nitrification, converts the nitrate nitrogen into gaseous nitrogen. This is accomplished by (other) heterotrophic bacteria naturally present in the wastewater. Using nitrate as an oxygen source, these bacteria consume carbon, thereby converting nitrates to gaseous nitrogen, but only under the molecular oxygen-depleted conditions in which the dissolved molecular oxygen concentration is below about 1 milligram per liter. When this condition is met, the bacteria generate gaseous nitrogen from the nitrates, and the nitrogen is released harmlessly into the atmosphere.
U.S. Pat. No. 5,318,699 is an example of a conventional system for nitrifying and denitrifying septic tank wastewater. In the system, nitrogenated wastewater, discharged from the aerobic soakaway (which is composed of gravel), seeps downwards and passes through a body of organic carbon. The carbon ostensibly is under anaerobic conditions, causing de-nitrification to occur. The resulting carbon dioxide and nitrogen gases migrate upwards through the soakaway and are discharged into the atmosphere.
Although this system may be effective in reducing nitrogen levels in wastewater, it has drawbacks. For example, molecular oxygen can migrate into the body of carbon, thereby interfering with de-nitrification. Particulate matter in the wastewater can be carried by the wastewater into the soakaway. The particulate matter, through silting, can clog porous channels in the soakaway and body of organic carbon, thereby causing channeling of the wastewater through the organic carbon. Channeling, in turn, can reduce the level of conversion of nitrogen species into nitrogen gas. That is, the wastewater passing from the carbon body into the surrounding water table can contain high levels of nitrogen species.
Other systems either re-circulate nitrified nitrate-laden effluent to the anoxic and carbon-laden front of the treatment process for de-nitrification or pass nitrified effluent through a post secondary treatment anoxic process with the addition of a carbon source, such as methanol to the process.